This invention relates to digital transmission systems for transmitting digital data over a variety of media. In particular, the invention relates to a VSB (vestigial sideband) digital transmission system suitable for ATV (advanced television) signals transmitted over NTSC channels including currently unusable or "taboo" NTSC channels. The ATV signal is a digital television signal which may represent a high resolution high definition television signal (HDTV) or several lower resolution images or other digital information. The lower resolution ATV signals require a lower bit rate so that several such signals may be included in one channel band. A high data rate cable mode supports two or more ATV signals in one 6 MHz channel, depending upon their resolution, whereas a lower data rate, but more robust, terrestrial mode supports one or more ATV signals in one 6 MHz channel (again depending upon resolution) with minimal interference to or from NTSC channel signals. It will be appreciated that the invention is equally applicable to a data only modem and is not restricted to television systems.
The VSB transmissions system comprises a series of data frames organized into a plurality of data segments of 832 symbols each. Each data frame has a first data field sync segment, followed by 312 data segments,and a second data field sync segment followed by another 312 data segments. Each data segment consists of a four symbol data segment sync, each symbol comprising two levels, followed by 828 data and FEC (forward error correction) symbols. The error correction consists of R-S (Reed-Solomon) parity bytes.
The symbol rate is 4.5/286 MHz.times.684 which is approximately 10.76 MHz. The first term, 4.5/286, is the NTSC horizontal scan rate. All transmitted signals are multilevel. The sync symbols are always 2 level (binary). In the terrestrial or broadcast mode, 8 level symbols, which are trellis coded to represent 2 information bits per symbol, are transmitted and each segment corresponds to one R-S correction block of 207 bytes or 1656 bits per segment. In the high data rate cable mode, 16 level symbols (4 bits per symbol) may be used and in that event, each segment corresponds to two R-S correction blocks of 207 bytes which yields 3312 bits per segment. These are referred to as trellis coded 8-VSB for the terrestrial mode and 16-VSB for the cable mode. Of course, lower level VSB (2, 4 and 8-non trellis VSB) may also be used for the cable mode.
The transmission is via suppressed carrier modulation. Three hundred and ten kilohertz from the lower band edge, a small DC pilot is added to the signal for use by the VSB receiver to facilitate carrier lock. All payload data is randomized to insure that random data is transmitted even when constant data is being supplied to the system. The data and FEC bytes are interleaved for added protection against burst errors.
In accordance with the present invention, the field sync signal is characterized by a relatively long pseudo-random number sequence followed by three relatively short pseudo-random number sequences with the middle of the short pseudo-random number sequences alternating in polarity in alternate fields. In a preferred embodiment, the relatively long pseudo-random number sequence is used as an equalizer training signal in terrestrial broadcast applications and one, of the relatively short pseudo-random number sequences is used as an equalizer training signal in more benign applications, such as in a cable environment. Equalizers using the shorter sequences require less hardware complexity.